The present invention relates to an improved method for treating subterranean formations with particulate material. The improved particulate material of this invention has utility, including but not limited to, use as a proppant in hydraulic fracturing use as a fluid loss agent in hydraulic facturing and as a screening material in gravel packing. The invention also relates to a method for producing improved particulate material for use in the production of shell molds and shell cores in the foundry industry.
In the completion and operation of oil wells, gas wells, water wells, and similar boreholes, it frequently is desirable to alter the producing characteristic of the formation by treating the well. Many such treatments involve the use of particulate material. For example, in hydraulic fracturing, particles (propping agents) are used to maintain the fracture in a propped condition. Smaller size particles (70 to 140 mesh) are used to control fluid loss during fracturing. Also in sand control techniques, particulate matter is placed in the well to prevent the influx or encroachment of formation sand or particles.
Although particulate material is used in the treatment of formations for a variety of reasons, there is one problem common among such treatments, the problem of particle stability. This problem can best be appreciated when considered in connection with specific treating techniques.
In hydraulic fracturing, propping agent particles under high closure stress tend to fragment and disintegrate. At closure stresses above about 5,000 p.s.i., silica sand, the most common proppant, is not normally employed due to its propensity to disintegrate. The resulting fines from this disintegration migrate and plug the interstitial flow passages in the propped interval. These migratory fines drastically reduce the permeability of the propped fracture.
Other propping agents have been used to increase well productivity. Organic materials, such as the shells of walnuts, coconuts and pecans have been used with some success. These organic materials are deformed rather than crushed when the fracture closes under the overburden load. Aluminum propping agents are another type of propping agent which deforms rather than fails under loading. While propping agents such as these avoid the problem of creating fines, they suffer the infirmity of allowing the propped fracture width to close as the propant is squeezed flatter and flatter with time. In addition, as these particles are squeezed flatter and flatter the spaces between the particles grow smaller. This combination of decreased fracture width and decreased space between the particles results in reduced flow capacity.
An improved proppant over the materials mentioned above is spherical pellets of high strength glass. These high strength glass proppants are vitreous, rigid, and have a high compressive strength which allows them to withstand overburden pressures of moderate magnitude. In addition, their uniform spherical shape aids in placing the particles and providing maximum flow through the fracture. While these beads have a high strength when employed in monolayers, they are less satisfactory in multilayer packs. In brine at 250.degree. F., the high strength glass beads have a tendency to disintegrate at stress levels between 5000 and 6000 p.s.i. with a resultant permeability which is no better, if not worse, than sand under comparable conditions.
Resin coated particles have been used in efforts to improve the stability of proppants at high closure stresses. Sand or other substrates have been coated with an infusible resin such as an epoxy or phenolic resin. These materials are superior to sand at intermediate stress levels. However, at high temperature and high stress levels, the resin coated particles still show a decrease in permeability to about the same degree as silica sand.
In gravel pack completions, particularly sized aggregate is placed in the well adjacent to the formation to form a filter bed through which produced fluids must flow. In one type of gravel packed completion, e.g. a linerless gravel pack, the aggregate material is injected through the well casing perforations to provide a filter outside the casing for each perforation. This type of completion frequently fails because of the inability of the aggregate to bridge across the perforation, with the result of the aggregate and formation sand entering the well bore.
Another type of gravel packed completion frequently used for sand control purposes is the liner gravel pack. This type of completion employs a well liner or screen packed in aggregate. Because of settling or migration of the aggregate it is frequently difficult to maintain the gravel in surrounding relation to the liner. Also, failure of the liner caused by corrosion or collapse results in the loss of the filter bed surrounding the liner, at least in the vicinity of the liner failure.
Obviously, a desirable characteristic of well completions involving the use of particulate material is one of particle stability. Efforts to provide such stability, particularly in gravel packed completions, include the use of organic resins or resinous materials.
U.S. Pat. No. 3,857,444 to Copeland discloses a method for forming a permeably consolidated gravel pack in a well bore. The slurry containing a particulate material coated with an uncured epoxy resin and a curing agent in a solvent is slurried in liquid hydrocarbon and introduced into place in the formation. The well is shut in until the resin coated particulate mass cures to form a permeable consolidated sand or gravel pack.
U.S. Pat. No. 3,929,191 to Graham et al discloses a method for producing coated particles for use in treating subterranean formations. The particles in this method are coated with a resin dissolved in a solvent which is then evaporated. This patent also discloses that the coating may be produced by mixing the particles with a melted resin and subsequently cooling the mixture, forming a coating of resin on the particles.
The Graham patent also discloses that the addition of coupling agents to the system improves the bonding of the resin to the particles. This improved bonding strength between the resin and particles increases the strength of the mass formed when the resin coated particles are fused and cured into a porous mass. This increased strength is important due to the high stresses the material may be subjected to in use such as when used as a proppant in hydraulic fracturing.